1. Field of the Invention
The invention relates to a process for thermal treatment of residual materials containing heavy metal and iron oxide.
2. Discussion of the Background
Large quantities of residual materials containing heavy metal and iron oxide in the form of dust or sludge are produced in the electric or converter steel mills. Residual materials are separated by waste gas cleaning plants from the waste gas as dust or sludge. The disposal of this residual material is expensive or the final storage of these materials is problematic. The sludges are generally stored in artificial lakes in the open, whereas the dusts are stored underground in tunnels.
Typical compositions of dusts and sludges containing heavy metal from electric or converter steel mills are shown in the following table.
U.S. Pat. No. 3,756,804 describes a process for the thermal treatment of waste gas dusts containing heavy metal and iron oxides in a multiple-hearth furnace incorporating several hearths one above the other. The waste gas dusts containing heavy metal and iron oxides are mixed with reduction agents, conveyed to the uppermost hearth of the multiple-hearth furnace and gradually transferred to the lower hearths. The reduction agent reacts with the waste gas dusts containing heavy metal and iron oxides so as to form heavy metals and directly reduced iron, the heavy metals being vaporised. A by-pass duct located in the furnace wall at the level of the topmost intermediate hearth conveys the gases to a condenser cooler, where the temperature of the gases is lowered so that the heavy metals condense and are deposited on plates in the condenser cooler. The gases released to a considerable extent from the vaporised heavy metals are subsequently heated and then introduced into the lower part of the furnace.
According to one aspect of the present invention, a process for thermal treatment of residual materials containing heavy metal and iron oxide, includes providing a multiple-hearth furnace having a plurality of hearths provided one above the other, depositing the residual materials containing heavy metal and iron oxide continuously on a top hearth of the plurality of hearths, gradually transferring the residual materials containing heavy metal and iron oxide to lower hearths of the plurality of hearths, introducing reducing agents to at least one of the plurality of hearths and reacting the residual materials containing heavy metal and iron oxide to form heavy metals and directly reduced iron, exhausting gases containing heavy metals from below hearths of the plurality of hearths where the heavy metals are being vaporised, re-injecting at least a part of the gases into the multiple-hearth furnace from above the hearths of the plurality of hearths where the heavy metals are being vaporised, and discharging the directly reduced iron together with residues of the reducing agents in an area of a bottom hearth of the plurality of hearths in the multiple hearth furnace.
According to another aspect of the present invention, a multiple-hearth furnace for thermal treatment of residual materials containing heavy metal and iron oxide, includes a plurality of hearths provided one above the other inside the multiple-hearth furnace, a depositing device for depositing the residual materials containing heavy metal and iron oxide continuously on a top hearth of the plurality of hearths, a transferring device for gradually transferring the residual materials containing heavy metal and iron oxide to lower hearths of the plurality of hearths, an introducing device for introducing reducing agents to at least one of the plurality of hearths and reacting the residual materials containing heavy metal and iron oxide to form heavy metals and directly reduced iron, an exhausting device for exhausting gases containing heavy metals from below hearths of the plurality of hearths where the heavy metals are being vaporised, a re-injecting means for re-injecting at least a part of the gases into the multiple-hearth furnace from above the hearths of the plurality of hearths where the heavy metals are being vaporised, and a discharging device for discharging the directly reduced iron together with residues of the reducing agents in an area of a bottom hearth of the plurality of hearths in the multiple hearth furnace.